1. Field of the Invention
This invention relates to a camera having an automatic focus adjusting apparatus, and in particular to such a camera which is provided with a control circuit which allows a focus adjusting operation only when the main object lies within a predetermined range relative to a distance value set by the photographer, thereby preventing malfunctioning of the camera.
2. Description of the Prior Art
Usually, an automatic focus adjusting apparatus is designed to drive a photo-taking lens and effect focusing with attention paid to the focus information (amount of image deviation, sharpness, etc.) of an object caught in the vicinity of the center of the photographing picture plane (hereinafter referred to as the object caught in the distance measuring view field). Consequently, this has led to a disadvantage that when an object desired by the photographer (hereinafter referred to as the main object) deviates from the center of the photographing picture plane and cannot be caught in the distance measuring view field, a focusing operation is executed with respect to an object of photography which differs from the main object caught in the distance measuring view field. Particularly, if an automatic focus adjusting apparatus is applied to a single lens reflex camera which often employs a lens of a long focus system, the following inconveniences will occur during the use of the long focus lens:
(1) Due to high photographing magnification, it is difficult to continue to catch the main object in the distance measuring view field and therefore, the above-noted disadvantage presents itself, remarkably; and
(2) The amount of lens detection is great and therefore, when an object which remarkably differs in distance from the main object is caught in the distance measuring view field, a long time is required for the focusing operation with respect to the object caught in the distance measuring view field and a long time is required before the focusing with respect to the main object is again effected, and this may lead to the missing of the photographing chance.
As a method for eliminating the above-noted disadvantages, there is known a method in which the photographer pre-divides the distance of the object to be photographed into a long distance zone, a medium distance zone and a short distance zone, pre-designates a predetermined one of these zones and effects a focusing operation only within the range of the designated zone.
That is, when the main object lies, for example, in the medium distance zone, the photographer predesignates the medium distance zone and catches the main object in the distance measuring view field and in such condition, effects a focusing operation, whereby even if the main object deviates from the distance measuring view field, the distance value with respect to which focusing has been effected is kept within the range of said zone to prevent a focusing operation from being effected at a distance value greatly deviating from the in-focus distance relative to the main object, thus ensuring a substantially allowable focus condition as a photograph for the main object.
According to this method, however, it is necessary to foresee in what zone the main object lies and it is not only necessary to make a judgment (distance forecast) contrary to the purpose of the focus adjusting apparatus directed to automatic distance adjustment, but also, where the main object lies on the boundary between the zones, accurate focusing can never be accomplished with respect to the main object if the main object moves even slightly when it is caught in the distance measuring view field.
That is, where the main object lies on the boundary between the long distance zone and the medium distance zone, if the long distance zone is designated, a focusing operation is allowed in the long distance zone when the main object deviates from the distance measuring view field and therefore, the lens can be prevented from being much out of focus with respect to the main object, but if the main object slightly moves into the medium distance zone when it is caught in the distance measuring view field, accurate focusing with respect to the main object can no longer be executed.